This invention relates to a magnetic recording and reproducing device and, more particularly, to saving of time required for entering the tracking of heads into tracking servo control during reproduction of a recorded signal in a magntic recording and reproducing device such as an R-DAT (rotary head type digital audio tape recorder).
The R-DAT is a device which converts analog signals such as an audio signal into PCM signals, records the PCM signals on a magnetic tape and reproduces the same.
As shown in FIG. 2, the device comprises a rotary head 2 having two magnetic heads A and B separated by 180 degree interval on the circumferential surface of a cylinder 1. A tape 3 is loaded from a casette housing 4 with a vertical post 5 or with an inclined post 6, wound on the circumferential surface of the rotary head 2 for 90 degrees, supported by a fixed guide 7, and run by a capstan 8 and a pinch roller 9.
The diameter of the rotary head 2 is 30 mm, and the winding angle for the tape is 90 degrees. For recording and reproduction, at the mode I which is usually used, the rate of the rotary head 2 is 2,000 rpm (circumferential speed: 3.14 m/sec) and the speed of the tape 3 at 8.15 mm/sec in the direction identical to that of the rotary head 2. The relative speed of the head 2 as against the tape 3 is 3. 13 m/sec.
The recording system with R-DAT is a helical scanning azimuth recording. Its tape format as shown in FIG. 3 defines with the track angle of 6.degree. 22' 59.5" and azimuth angle of .+-.20.degree. with the tracks alternately traced by two heads A and B.
FIG. 4 shows a track format wherein audio data are recorded at the center of a PCM region, and sub-codes and control signals such as ATF (automantic track finding) are recorded on both sides thereof.
The PCM region comprises 128 blocks as shown in FIG. 5 each of which has recording regions for a block synchronizing (indicating the starting position of the block), ID (identification) code, block address, parity check code, and audio data. At the mode I, audio data uses 2's complement codes of quantization bit of 16 bits of the sampling frequency of 48 kHz, and the PCM data is divided into 8 bits in higher order and 8 bits of lower order, modulated from 8 bits into 10 bits (8-10 modulation) and recorded in 10 bits.
For tracking control during reproducing in the R-DAT, an automatic tracking system with ATF is employed. ATF system detects and compares crosstalks from two adjacent tracks by ATF signals recorded on the tracks, and controls the speed of the capstan motor for running tape so as to make crosstalks identical.
The principle of ATF will now be described below.
ATF signals are recorded at two locations on one track, i.e., ATF1 and AFT2, as shown in FIG. 4. As shown in the format in FIG. 6, pilot signal f1 and synchronizing signal f2 (or f3) are recorded on each track. The frequencies are specified respectively;
f1 =130.67 kHz PA1 f2 =522.67 kHz PA1 f3 =784.00 kHz
The frequency of f1 is low enough not to have much azimuth loss. The head A traces the track (hereinafter referred to as A track) of the synchronizing signals of f2. The head B traces the track (hereinafter referred to as B track) of the synchronizing signals of f3. The track length of a synchronizing signal differs between an odd number frame and an even number frame, which are defined as one (1) block and 0.5 block respectively.
If it is assumed that the head A is tracing the A track T4, pilot signals f1 of adjacent tracks T3 and T5 are obtained from the head A due to crosstalks in addition to a reproduced signal from the A track T4. This is because the head has a width 1.5 times as large as a track. If the head A is tracing the A track T4 correctly, crosstalks from the B tracks T3 and T5 become equal, but if the head is deviated to either direction, the crosstalks from them become different from each other. The crosstalks from the adjacent B tracks T5 and T3 are detected by detecting amplitude levels of the pilot signal f1 on the B track T5 and of the pilot signal f1 of the track T3 at the detection timing of the synchronizing signal f2 of the track T4. The difference in the crosstalks therefore is presumed to represent tracking errors.
FIG. 7 shows a prior art ATF device based on the above mentioned principle. A reproduced signal from the head A is applied to a low-pass filter 16 via a reproduction amplifier 14 to extract pilot signal f1. The extracted pilot signal f1 is applied to a tracking error detection circuit 21, detected in envelope by an envelope detection circuit 18, and applied to a sample hold circuit 26. A synchronizing signal detector 19 detects synchronizing signal f2 with an equalizer 20 and a comparator 22, and the comparator 22 outptus "1" during the period when the synchronzing signal is being detected.
A logic circuit 24 outputs sample hold signals SP1 and SP2 at the timing of the detected synchronizing signal f2. As the sample hold signal SP1 is provided at a timing immediately after the start of detection of the synchronizing signal f2, if the output from the circuit 18 is sampled with the SP1, a sample hold circuit 26 can hold crosstalk amplitude level of the pilot signal f1 on the B track T5 which is the track next to the A track T4 currently being traced. A subtractor 28 implements subtraction between the output of the sample hold circuit 26 and the output of the detector 18. A sample hold signal SP2 is produced after the time equivalent to 2 blocks after the start of detection of the synchronizing signals f2 (timing substantially at the center of the pilot signal f1 on the B track T3). By sample holding the output from the subtractor 28 with this signal SP2, the difference in crosstalk amplitude levels between the pilot signals f1 of the track T3 and of the track T5 positioned on both sides of the track T4 which is currently being traced is sample held at the circuit 30.
The output from the circuit 30 is applied to a capstan servo circuit 34 as a tracking error signal. The circuit 34 controls the speed of the capstan motor 36 in such a manner that the tracking error becomes zero. This eventually controls the running speed of the tape 10 thereby correcting the tracking error.
In the ATF device shown in FIG. 7, when the head A is deviated to the left from the center, crosstalks of the pilot signal from the B track T3 increases. A negative signal is therefore held at the sample hold circuit 30. In order to correct this, the capstan motor 36 increases the speed.
When the head A is deviated to the right, the crosstalk of the pilot signal from the B track T5 increases, and a positive singal is held at the circuit 30. For correcting this, the speed of the capstan motor 36 is decreased. Accordingly, if tracing of the head A is substantially correct as shown by I in FIG. 8, tracking control is made so that the tracking will be made over the A track as shown by arrows a, a.
Similarly in the head B, crosstalk signals of left and right adjacent tracks are detected on the basis of the detection timing of the synchronizing signal f3 of the B track and the speed of the capstan motor 36 is controlled so as to reduce the difference between the two crosstalk signals to zero.
As described above, in the R-DAT, the tracking control is carried out by differring the frequencies of the synchronizing signals of ATF from each other, namely f2 for the A track and f3 for the B track, so as to cause the head A to trace the A track and the head B to trace the B track.
Such tracking control depending simply upon difference in the frequencies of synchronizing signals however poses a problem. That is, if the head trace has been deviated by about one track between the start and end of reproduction as shown by II in FIG. 8, the head A detects the synchronizing signal f2 in different A tracks at positions ATF1 and ATF2. The tracking control in this case would be made in such a manner that the head A is driven in opposite directions at the positions ATF1 and ATF2 as shown by arrows b, b in FIG. 8 so that forces acting on the head A would be balanced and the head trace would then be stabilized at this balanced state. In this state, however, a mistracking occurs since the head A mainly reproduces recorded signals from the B track in about the center of the track length.
For preventing occurrence of such state, distinction between the odd frame and the even frame is provided as shown in FIG. 6, each frame consisting of 2 tracks. In the odd frame, the synchronizing signal f2 or f3 has a length of one block whereas in the even frame it has a length of 0.5 block. The LSB (least significant bit) of the block address (FIG. 5) of each block in the PCM region is determined to be "1" in the odd block and "0" in the even block. Indication of the odd frame and even frame is issued at each single rotation of the rotary head which is equivalent to one frame. If the distinction of the odd frame and even frame detected by the length of the synchronizing signal f2 or f3 or the block address coincides with this indication, a tracking error is detected by utilizing this synchronizing signal f2 or f3 whereas if the distinction does not coincide with the indication, the synchronizing signal f2 or f3 is discarded so that a tracking error is not detected.
More specifically, in FIG. 7, an odd/even detection circuit 11 detects the length of the synchronizing signal f2 or f3, judging that the frame is the odd frame if the length of the synchronizing signal is one block and it is the even frame if the length thereof is 0.5 block. As the rotary head is rotated, a PG (phase generator) generates one shot of pulse during a period of time between the end of reproduction by the head B (more exactly the end of the region of ATF2) and the start of reproduction by the head A (more exactly the start of the region of ATF1). This pulse is applied to a flip-flop circuit 13 to set or reset this circuit 13 steadily during each rotation of the rotation head. The Q output "1" or "0" of the flip-flop circuit 13 designates the odd number or even number of the frame. A comparison circuit 15 controls the logic circuit 24 in such a manner that the logic circuit 24 produces a sample hold signal SP2 when the designation of the odd number or even number by the flip-flop circuit 13 coincides with the odd number or even number detected by the odd/even detection circuit 11 whereas the logic circuit 24 does not produce the sample hold signal SP2 when these designation and detection do not coincide with each other. Further, if the sample hold signal SP2 is not produced for a predetermined period of time after the above described control by the comparison circuit 15, a switch 17 is switched to the b side to shift the tracking compulsorily to a position at which the sample hold signal SP2 is produced. Accordingly, if the head trace becomes the state shown by II in FIG. 8, the detection of the odd number and even number becomes opposite at the ATF1 region and ATF2 region while the designation of the odd number and even number remains unchanged between the ATF1 region and ATF2 region so that the detection of the odd number and even number becomes different from the designation either in the ATF1 region or ATF2 region. Accordingly, a tracking error in either the ATF1 region or ATF2 region in which the designation and detection coincide with each other is employed with a result that the tracking is corrected so as to cause the head A to trace the A track in only the ATF region in which the designation and detection coincide with each other. As a result of this tracking, the ATF synchronizing signal of the A track becomes reproducible after all in the ATF region in which the designation and detection do not coincide with each other so that the designation comes to coincide with the detection and the tracking thereby comes to be performed by the two tracking error signals.
In the prior art device in which frames are divided into odd number frames and even number frames and either frame is designated, there arises a case in which, despite the fact that the head A is tracing the A track and the head B is tracing the B track as shown by state I in FIG. 8 so that reproduction is possible, neither the tracking error signal ATF1 nor ATF2 is employed for a period of time when the tracking is shifted to compulsorily cause the designation of the odd number or even number to coincide with the detection thereof in case they do not coincide with each other with a result that the tracking is not subjected to tracking servo control for a relatively long period of time in a state where, for example, the operation mode is changed from a stop mode or pause mode to a play mode or in a portion of junction of recording in which the alternate continuity of the odd and even number frames does not exist.
It is, therefore, an object of the invention to provide a magnetic recording and reproducing device which has eliminated the above described problem of the prior art magnetic recording and reproducing device and has realized saving of time required for subjecting the tracking of the heads to tracking servo control.